The present invention relates to limited slip differentials (LSDs), and more particularly, to such differentials of the type in which the means for retarding differentiating action may be modulated in response to an external input signal, and especially, an electrical input signal.
Limited slip differentials of the type to which the present invention relates typically include a gear case defining a gear chamber, and disposed therein, a differential gear set including at least one input pinion gear, and a pair of output side gears. A clutch pack is typically disposed between at least one of the side gears and an adjacent surface of the gear case, such that the clutch pack is operable to retard, or even prevent, relative rotation between the gear case and the one side gear. Although the present invention is not limited only to limited slip differentials in which the clutch pack is disposed axially between one of the side gears and an adjacent surface of the gear case, the present invention is especially advantageous when used in connection with an LSD having such an arrangement, and the invention will be described in connection therewith.
In many LSDs, some sort of actuating mechanism is provided to actuate or move the clutch pack from its disengaged condition to an engaged condition. One of the current trends in the field of vehicle traction modifiers involves the need to be able to actuate the clutch pack in response to an external input signal, rather than for the clutch pack to be actuated only in response to the sensing of a predetermined speed differential, as has typically been the case in the prior art.
One prior art approach to providing the ability to modulate an LSD has been by imposing a load on a clutch pack in response to the ramping of a ball-ramp actuator. In U.S. Pat. No. 4,805,486, the ball-ramp actuator is disposed externally to the differential case, and ramping of the actuator is initiated by rotation of a servomotor, and by means of a parallel-shaft gear train, thus requiring substantial redesign of the surrounding differential housing.
In U.S. Pat. Nos. 5,019,021 and 5,092,825, both of which are assigned to the assignee of the present invention and incorporated herein by reference, the ball-ramp actuator is built into the end wall of the differential housing. The ball-ramp actuator includes one actuating plate disposed within the differential housing, and another actuating plate disposed external to the differential housing. Each of the actuating plates includes appropriate ramps, and the cam balls engaging the ramps are disposed in openings defined by the end wall of the differential housing.
The above-incorporated patents provide a limited slip differential which is fairly simple and compact, and wherein the presence of the ball-ramp actuator for the clutch pack does not require substantial redesign of the differential.
However, in the above-incorporated patents, the actuator arrangement for initiating ramp-up of the ball-ramp actuator involves the use of an electromagnetic device operable to frictionally engage the outer periphery of the actuating plate external to the differential housing, thus retarding its rotation, relative to the differential, to initiate ramp up. It will be appreciated by those skilled in the art that frictional engagement of the actuating plate, at one location on its periphery, will result in an unbalanced radial load on both the actuating plate and the differential generally.
In addition, the arrangement of the incorporated patents provides relatively little frictional engagement area, thus necessitating an undesirably high engagement force in order to achieve the required retarding of the actuating plate. However, such a high engagement force is difficult to achieve in the arrangement described, because it involves a fairly long travel of the electromagnetic device, and, as is well known to those skilled in the art, force decreases as travelled distance increases. Another related problem is the difficulty of controlling the air gap between the actuator and the actuating plate periphery, and as is also well known, variations in the air gap cause major variations in performance.
Finally, the arrangement described requires an actuator housing for the electromagnetic device, with the housing being disposed transversely offset from the axis of the differential. Such an arrangement requires that the outer differential housing be modified substantially to accommodate the actuator housing, which adds another source of error in regard to the location of the actuator, relative to the actuating plate periphery.